Antagonise English eccentric (8)
I believe the answer is:
estrange
'antagonise' is the definition.
(I've seen this before)
'english eccentric' is the wordplay.
'english' becomes 'e' (abbreviation).
'eccentric' becomes 'strange' (synonyms).
'e'+'strange'='ESTRANGE'
(Other definitions for estrange that I've seen before include "turn away?" , "Antagonise using reagents" , "Anger set (anag.)" , "Lose friendship" , "Cause to become unfriendly using reagents" .)